ADSL2+ connection dropouts and huge speed drop

I connected a new TPG ADSL2+ service in my apartment in Hawthorn, Victoria approx. 3 months ago. Up until late June 2018, everything was fine, and I was regularly getting decent speeds of 13-14Mbps (I live just over 1km from the exchange) and no dropouts.

However, around that time, I decided to finally get a home phone.

Ever since I plugged in a splitter and connected both the modem/router and phone to the line, I have noticed that:-

3. The phone line/connection sounds very distorted when I go to make a call (it is a cordless phone though).

I have tried unplugging the phone and splitter and just connecting the modem/router back directly into the line and the same problems keeps happening! I'm at my wits end!! I've spent hours rebooting, reconnecting, plugging, unplugging and nothing has made any difference. I don't know what to do now.

Are you guys able to check my connection to see if you can figure out what's going on?